Personality in Bonobos

Psychological Science2015, Vol. 26(9) 1430 –1439© The Author(s) 2015Reprints and permissions: sagepub.com/journalsPermissions.navDOI: 10.1177/0956797615589933pss.sagepub.com

Research Article

Studying humans’ closely related nonhuman relatives—the great apes—helps in understanding the evolution of complex behavioral phenomena. One recent application of this approach has been to study the organization of personality traits into dimensions (e.g., Gosling & Graybeal, 2007), with the goal of identifying which per-sonality dimensions are ancestral (i.e., descended from a common ancestor species) and which are derived (i.e., recently evolved in response to ecological or social chal-lenges faced by a given species).

Humans and chimpanzees share a common ancestor that lived 6 million years ago (Glazko & Nei, 2003). A study of chimpanzees whose personalities were rated by zoo keepers and volunteer research assistants found six personality dimensions (King & Figueredo, 1997). Dominance, the first dimension identified in that study, consisted of traits related to competitive prowess; the remaining five dimensions identified in that study

resembled the domains of the human five-factor model (or Big Five)—extraversion, conscientiousness, agree-ableness, neuroticism, and openness (Goldberg, 1990; McCrae & Costa, 1999). Broadly speaking, most or all of these dimensions have been identified in studies of chim-panzees living in environments other than zoos (King, Weiss, & Farmer, 2005; Weiss, King, & Hopkins, 2007) and in studies using different rating forms (Dutton, 2008; Freeman et al., 2013).

589933 PSSXXX10.1177/0956797615589933Weiss et al.Bonobo Personalityresearch-article2015

Corresponding Authors:Alexander Weiss, Department of Psychology, School of Philosophy, Psychology and Language Sciences, The University of Edinburgh, Edinburgh EH8 9JZ, United Kingdom E-mail: [email protected]

Nicky Staes, Ethology Research Group, Department of Biology, The University of Antwerp, 2610, Antwerp, Belgium E-mail: [email protected]

Personality in Bonobos

Alexander Weiss1,2, Nicky Staes3,4, Jeffrey J. M. Pereboom4, Miho Inoue-Murayama5, Jeroen M. G. Stevens3,4, and Marcel Eens3

1Department of Psychology, School of Philosophy, Psychology and Language Sciences, The University of Edinburgh; 2Scottish Primate Research Group; 3Ethology Research Group, Department of Biology, The University of Antwerp; 4Centre for Research and Conservation, Royal Zoological Society of Antwerp; and 5Wildlife Research Center of Kyoto University

AbstractTo better understand human and chimpanzee personality evolution, we obtained trait ratings of personality for 154 captive bonobos (~80% of the U.S. and European population). We found factors that we labeled Assertiveness, Conscientiousness, Openness, Agreeableness, Attentiveness, and Extraversion. The interrater reliabilities and test-retest reliabilities for these factors were comparable to those found in humans and other species. Using orthogonal targeted Procrustes rotations, we compared the bonobo dimensions with those of three samples of captive chimpanzees. Overall congruence coefficients indicated a fair degree of similarity; at the factor level, there was good evidence for Assertiveness, Conscientiousness, Openness, and Agreeableness in the chimpanzee samples; evidence for Attentiveness and Extraversion was poor. These findings suggest that, as expected given their close phylogenetic relationship, bonobo personality structure resembles chimpanzee personality structure in some respects. However, divergent evolution, perhaps as a result of socioecological differences between bonobos and chimpanzees, also appears to have shaped personality structure in these species.

Keywordsbehavior, chimpanzee, evolution, Pan paniscus, Pan troglodytes, primate

Received 9/19/14; Revision accepted 5/14/15

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Bonobo Personality 1431

We sought to better understand chimpanzee and human personality evolution by assessing personality in bonobos. The common ancestor of bonobos and chim-panzees lived about 1 million years ago, and thus bono-bos share a common ancestor with humans and chimpanzees (Glazko & Nei, 2003). Bonobos are closely related to chimpanzees and, like chimpanzees, live in fission-fusion societies in which females are the dispers-ing sex (Furuichi, 2011). Nonetheless, bonobos differ from chimpanzees in several key respects. First, com-pared with chimpanzees, bonobos live in more stable and predictable environments with low seasonality and more stable food availability (Furuichi, 2011). This is believed to have led bonobos to have a lower degree of fission-fusion and lower rates of inter- and intragroup competition than chimpanzees do (Furuichi, 2011). Second, unlike chimpanzees, bonobos exhibit nonex-clusive female dominance (Furuichi, 2011; Vervaecke, De Vries, & Van Elsacker, 2000); bonobo females form stable and valuable social relationships (Stevens, de Groot, & Staes, 2015). Third, compared with chimpan-zees, bonobos are more risk-averse, are better at socio-cognitive tasks than at instrumental tasks, and are less bold (see Hare, Wobber, & Wrangham, 2012, for a review).

In the present study, we compared the personality structure of captive bonobos with personality structures previously identified in three samples of captive chim-panzees (King & Figueredo, 1997; Weiss et al., 2007, 2009). Personality dimensions shared by these species are likely to be ancestral and to reflect their close phylo-genetic relatedness (Glazko & Nei, 2003) and their similar social structures (Furuichi, 2011). Personality dimensions that are not shared are likely to be derived (i.e., to have recently evolved in chimpanzees, bonobos, or both). These derived characteristics could reflect genetic drift or differences between the two species, specifically the dif-ferences in the rates of inter- and intragroup competition and how dominance is expressed (Furuichi, 2011; Vervaecke et al., 2000).

Method

Subjects

The subjects were all noninfant bonobos from 16 facili-ties that participated in the study (7 zoological parks and 1 research institute in the United States, 5 zoological parks in Germany, 1 zoological park in Belgium, 1 zoo-logical park in the United Kingdom, and 1 zoological park in the Netherlands). The subjects comprised about 80% of the current captive population in Europe and the United States and consisted of 71 males (mean age = 16.2 years, SD = 10.8, age range = 1.6–43.9 years) and

83 females (mean age = 16.6 years, SD = 12.7, age range = 2.2–61.5 years). This population originated from 36 wild-caught bonobos that belonged to at least four populations spread across bonobos’ range (Reinartz, 1997). Therefore, it is likely that the variation within our sample is within what one would see in the wild, albeit at the lower end of that range.

Personality assessment

Questionnaire. Personality was assessed via ratings on the Hominoid Personality Questionnaire (HPQ; Weiss et al., 2009).1 This questionnaire consists of 54 personality-descriptive adjectives, each paired with one to three descriptive sentences that set the adjective in the context of behavior. The HPQ instructs raters to make ratings on a 7-point scale (1 = displays total absence or negligible amounts of the trait, 7 = displays extremely large amounts of the trait) and to not discuss their ratings.

Of the 54 items, 43 were originally developed and used to assess chimpanzee personality (King & Figueredo, 1997). Of these items, 41 were derived from a taxonomy of the Big Five (Goldberg, 1990) and 2 were devised for rating chimpanzees. The remaining items included 5 that represented openness and neuroticism, added as part of a study of orangutan personality (Weiss, King, & Perkins, 2006), and 6 that represented openness and conscien-tiousness, added as part of a later study on chimpanzees (Weiss et al., 2009).

Raters at the German and Dutch facilities completed versions of the HPQ in their native languages. After the data had been collected, a researcher not involved in the study found a minor error in the German translation of the HPQ: The behavioral descriptive sentence for the adjective “autistic” refers to “rocking” in the English ver-sion and “stone throwing” in the German version. Because the behavioral descriptive sentences are offered as a guide and not as definitive descriptions of the traits, and because this difference would lead our results to be more conservative, we did not exclude this item.

Raters and ratings. Caretakers and staff at zoological parks, researchers, and other people who knew the indi-vidual subjects made the ratings. Some of the ratings did not include information regarding the raters’ identities; we considered the seven unidentified ratings (each for a different subject) at one zoo to have come from a single rater and the six unidentified ratings (each for a different subject) at another zoo to have come from a different single rater. Furthermore, at one zoo, two raters assessed a single subject together. These two raters and a third worked together to rate 2 additional subjects. In these cases, joint ratings were considered to have come from a single rater.



1432 Weiss et al.

Of the subjects in the sample, 112 subjects were rated only one time; 47 were rated by 1 rater, and 65 were rated by 2 to 5 raters. The remaining 42 subjects were rated two times; the second rating occurred an average of 5.6 years (SD = 0.4) after the first rating. Of these 42 sub-jects, 2 were rated by 1 rater, and 40 were rated by 2, 3, or 5 raters; 17 were rated by the same rater or raters dur-ing both data-collection waves. There were 25 and 29 raters in the first and second data-collection waves, respectively. Five raters rated subjects in both waves.

The length of time that raters had known the subjects was available for 222 of the 291 ratings in the first data-collection wave (M = 5.8 years, SD = 5.3) and for 103 of 117 ratings in the second data-collection wave (M = 5.6 years, SD = 6.5).

Analyses

Interrater reliabilities, internal consistencies, and test-retest reliabilities. Interrater reliabilities of the HPQ items and personality factors were calculated using intraclass correlations (ICCs). ICC(3,1) indicates the reli-ability of single ratings; ICC(3,k) indicates the reliability of the mean scores of an average of k raters (Shrout & Fleiss, 1979). For each factor, we determined the internal consistency reliabilities (Cronbach’s alphas) of the mean item scores across raters using the alpha function of the psych package for R (Revelle, 2015). For subjects that were rated twice, we computed the test-retest reliabilities of factors using the rpt.aov function in R (Nakagawa & Schielzeth, 2010).

Data reduction. We performed an unweighted least squares factor analysis using the fa function of the psych package (Revelle, 2015) for the R software environment (Version 3.0.2; R Development Core Team, 2013). We used this analysis to determine the structure of all items with interrater reliabilities greater than zero. As in previ-ous studies (e.g., Weiss, Adams, Widdig, & Gerald, 2011), we used the wave of data with the most responses; for subjects rated by multiple raters, we took the mean rating across raters. We determined the number of factors to extract by examining the scree plot and by conducting a parallel analysis via the fa.parallel function of the psych package for R (Revelle, 2015). Next, to determine which type of rotation would be appropriate, we conducted both orthogonal (varimax) and oblique (promax) rota-tions. If the intercorrelations between promax-rotated factors were high and the structure of the promax-rotated factors differed from that of the varimax-rotated factors, we retained the promax-rotated factors. If the factor intercorrelations were not high and the structures did not differ, we retained the varimax-rotated factors. As in pre-vious studies (e.g., Weiss et al., 2006), for the purpose of

labeling factors and creating unit-weighted factor scores, we defined salient loadings as those that were equal to or greater than |.4|. When an item had multiple salient loadings, we interpreted an item as belonging to the fac-tor on which it had the highest loading.

Cross-species comparisons. As we noted in the intro-duction, prior studies using the HPQ or its predecessor, the Chimpanzee Personality Questionnaire, revealed that chimpanzee personality can be described along six dimensions (King & Figueredo, 1997; Weiss et al., 2007, 2009). We compared structures derived from three chim-panzee samples with the bonobo personality structure. The first chimpanzee sample, the ChimpanZoo sample, comprised 100 chimpanzees housed in U.S. zoos, as described previously (King & Figueredo, 1997). The sec-ond chimpanzee sample, the Yerkes sample, comprised 175 chimpanzees housed in Yerkes National Primate Research Center, as described previously (Weiss et al., 2007). Ratings of the individuals in both of these samples were made using the original 43-adjective questionnaire (King & Figueredo, 1997). The third chimpanzee sample, the Japanese sample, comprised 157 individuals housed in zoos, research centers, and a sanctuary, all located in Japan. Of these subjects, 146 were described in a previ-ous study (Weiss et al., 2009). The 11 additional subjects included 4 that were housed in the Fukuoka Zoo, 5 that were housed in the Higashiyama Zoo, and 2 that were housed in the Hirakawa Zoo. All chimpanzees in the third sample were rated using a Japanese translation of the HPQ (Weiss et al., 2009).

We planned to use different approaches to compare bonobos and chimpanzees depending on whether we found that bonobos, like chimpanzees, have six person-ality dimensions (King & Figueredo, 1997). If we had found that bonobos and chimpanzees have a different number of personality dimensions, we would have used a method described in previous studies (e.g., Weiss et al., 2011): We would first have computed unit-weighted scores on the basis of the bonobo and chimpanzee struc-tures and then obtained correlations between these scores. However, because we found that the bonobos have six personality dimensions, we compared the chim-panzee and bonobo structures using targeted orthogonal Procrustes rotations (McCrae, Zonderman, Bond, Costa, & Paunonen, 1996). Each rotation compared a structure derived from one of the three chimpanzee samples with a structure derived from the present bonobo sample. The chimpanzee studies differed with respect to whether fac-tors or components were extracted and in the number of items subjected to data reduction. We therefore made sure that, for each targeted rotation, the extraction method and the set of items used to derive the bonobo target matrix matched that of the chimpanzee sample. We




interpreted congruence coefficients using guidelines derived from a study in which experts judged the similar-ity between structures with different levels of congruence (Lorenzo-Seva & ten Berge, 2006). The researchers found that congruence coefficients below .85 were not indica-tive of factor similarity, that congruence coefficients rang-ing from .85 to .94 were indicative of “fair similarity,” and that congruence coefficients greater than or equal to .95 were indicative of equality and thus indicated “good simi-larity” (Lorenzo-Seva & ten Berge, 2006, p. 61).

Results

Interrater reliabilities of items

Interrater reliabilities for the 54 adjectives for each rating wave are presented in Table S1 in the Supplemental Material available online. In the first wave, ICC(3,1) val-ues ranged from .04 for “sensitive” to .70 for “submissive” (M = .34, SD = .16), and ICC(3,k) values ranged from .11 for “sensitive” to .86 for “submissive” (M = .55, SD = .18). For the second wave, ICC(3,1) values ranged from .09 for “erratic” to .82 for “dominant” (M = .45, SD = .13), and ICC(3,k) values ranged from .21 for “erratic” to .93 for “dominant” (M = .69, SD = .12). Because all items had reliabilities greater than zero for both waves, we did not exclude any items.

Personality structure

The scree plot and the parallel analysis indicated that there were six factors. The varimax- and promax-rotated factors did not differ appreciably, and the absolute factor intercor-relations were modest (M = .14, SD = .12). We therefore interpreted varimax-rotated factors (for the promax-rotated structure and factor intercorrelations, see Tables S2 and S3 in the Supplemental Material). These six factors explained 56% of the variance.

For ease of interpretation and to maintain consistency with previous studies in how dimensions are labeled, before interpreting factors, we multiplied the loadings of four factors by −1. On the basis of the items that loaded on each factor, we labeled the factors Assertiveness, Con-scientiousness, Openness, Attentiveness, Agreeableness, and Extraversion (see Table 1).

Assertiveness in bonobos resembled the dominance dimension in chimpanzees (King & Figueredo, 1997; Weiss et al., 2007, 2009), orangutans (Weiss et al., 2006), rhesus macaques (Weiss et al., 2011), and gorillas (Eckardt et al., 2015; Gold & Maple, 1994; Schaefer & Steklis, 2014). It also resembled the confidence component in Hanuman langurs (Konecná et al., 2008) and Barbary macaques (Konecná, Weiss, Lhota, & Wallner, 2012) and the asser-tiveness component in brown capuchin monkeys (Morton

et al., 2013). With respect to the five-factor model, this dimension resembled the assertiveness facet of extraver-sion in humans (Costa & McCrae, 1992). Bonobos with higher scores for this factor tended to be independent and confident, and more prone to displacing, threatening, or taking food from other bonobos. On the other hand, bonobos with lower scores for this factor tended to be less confident, more anxious, and more prone to getting displaced and physically hurt or emotionally upset as a result of agonistic interactions.

Conscientiousness in bonobos resembled conscien-tiousness in chimpanzees (King & Figueredo, 1997; Weiss et al., 2007, 2009) in that it was made up of two clusters (King, Weiss, & Sisco, 2008). It also partly overlapped the proto-agreeableness component in mountain gorillas (Eckardt et al., 2015), and it was similar to the conscien-tiousness facet of deliberation in the five-factor model (Costa & McCrae, 1992). Bonobos with high scores for this factor thus tended to be kind, considerate, and pre-dictable in their relationships with others, whereas bono-bos with low scores for this factor tended to be impulsive, unpredictable, and aggressive.

Openness in bonobos resembled its counterparts in humans (Costa & McCrae, 1992), chimpanzees (King & Figueredo, 1997; Weiss et al., 2007, 2009), rhesus macaques (Weiss et al., 2011), brown capuchin monkeys (Morton et al., 2013), and mountain gorillas (Eckardt et al., 2015). However, as in mountain gorillas (Eckardt et al., 2015) and brown capuchin monkeys (Morton et al., 2013), traits such as “active” and “playful” had pos-itive loadings on Openness in bonobos. These traits are associated with the activity facet of extraversion in humans (Costa & McCrae, 1992), the extraversion com-ponent in orangutans (Weiss et al., 2006), and the activ-ity component in rhesus macaques (Weiss et al., 2011). Bonobos with high scores for this factor thus tended to be playful and to engage actively in exploring novel social and physical environments. On the other hand, bonobos with low scores for this factor tended to be indolent, inactive, and disinclined to explore novel indi-viduals or environments.

Attentiveness in bonobos resembled the identically named component identified in brown capuchin mon-keys (Morton et al., 2013). The items loading on this fac-tor overlapped some with the items loading on the intellect component in orangutans (Weiss et al., 2006). In chimpanzees (King & Figueredo, 1997; Weiss et al., 2007, 2009), these items defined high dominance and high con-scientiousness. In gorillas, these items defined high dom-inance (Eckardt et al., 2015; Schaefer & Steklis, 2014). In rhesus macaques, these items defined high confidence, friendliness, and activity (Weiss et al., 2011). The Attentiveness factor was also similar to the conscientious-ness facets of order and self-discipline in the five-factor



1434 Weiss et al.

Table 1. Varimax-Rotated Factor Loadings

Adjective

Factor

h2Assertivenessa Conscientiousnessa Openness Attentivenessa Agreeableness Extraversiona

Anxious –.81 –.04 –.16 –.07 –.03 –.17 .71Timid –.76 .27 –.12 –.07 –.05 –.12 .69Fearful –.75 .06 –.17 –.16 –.10 –.12 .64Independent .71 –.20 –.02 .13 .04 –.30 .66Dominant .68 –.52 –.26 .07 .11 .07 .82Vulnerable –.66 .22 .10 .00 –.19 –.37 .67Submissive –.64 .43 .18 –.08 .04 –.17 .66Cool .63 .13 –.04 .08 .26 –.05 .50Stable .62 .24 .13 .11 .20 .03 .52Dependent –.61 .12 .17 –.37 .20 .35 .71Decisive .54 –.07 .09 .40 .15 .00 .49Persistent .52 –.40 .15 .14 .13 .05 .49Excitable –.52 –.30 –.05 –.12 .00 –.01 .39Cautious –.36 .31 –.10 .20 .35 –.08 .41Quitting –.20 .08 –.19 –.20 .12 .00 .14Bullying .27 –.76 .01 .08 –.16 .06 .68Aggressive .15 –.75 .10 .08 –.14 .05 .63Stingy .20 –.72 .02 –.14 –.03 –.05 .58Irritable –.12 –.70 –.17 –.09 –.14 –.21 .60Jealous –.04 –.68 .18 –.15 –.07 –.08 .53Gentle –.02 .67 –.02 .04 .53 –.06 .73Erratic –.29 –.60 –.02 –.29 –.06 –.18 .57Defiant .11 –.59 .23 –.11 –.19 –.05 .46Reckless .07 –.58 .19 –.38 –.21 –.05 .57Manipulative .21 –.55 .05 .17 .14 .14 .42Impulsive –.29 –.48 .28 –.15 –.14 –.06 .45Predictable .21 .41 –.23 .00 .26 –.05 .33Active –.13 –.12 .77 –.08 .09 .21 .68Playful –.08 –.05 .73 –.04 .10 .20 .59Inquisitive .22 –.01 .68 .21 .11 .03 .57Inventive .34 –.05 .64 .20 .03 –.24 .62Imitative –.08 .04 .63 –.22 .17 .13 .50Innovative .33 .01 .63 .07 .05 –.21 .56Conventional –.08 .16 –.61 .09 .31 –.01 .51Curious .15 –.14 .58 .23 .17 –.07 .46Lazy .12 .03 –.54 –.37 .06 –.30 .54Unemotional .15 .15 –.25 –.11 .12 –.01 .14Disorganized –.18 –.15 –.03 –.71 –.16 –.20 .63Intelligent .18 –.07 .15 .68 .34 .06 .65Clumsy .04 .00 –.04 –.68 –.06 –.15 .50Thoughtless –.18 –.17 –.09 –.68 –.18 –.03 .56Distractible –.22 –.11 .06 –.59 –.05 .12 .42Unperceptive –.33 –.15 –.16 –.50 .09 –.24 .47Friendly –.01 .40 .18 –.12 .71 .24 .77Affectionate .21 .28 .18 .08 .71 .32 .76Protective .30 –.04 –.13 .27 .70 –.02 .66Sympathetic .07 .36 .00 .17 .62 –.13 .56Helpful .07 .15 .17 .31 .59 .04 .51Sociable .15 –.06 .34 –.01 .56 .34 .57Sensitive .25 .15 –.15 .48 .53 –.03 .62Solitary –.17 .17 –.26 –.09 –.14 –.67 .61Depressed –.42 –.13 –.18 –.21 –.07 –.67 .72Autistic –.43 –.23 –.08 –.34 –.06 –.43 .55Individualistic .14 –.19 .23 –.18 –.04 –.43 .33 Proportion of variance

accounted for.14 .12 .09 .08 .08 .05

Note: Principal component analyses yielded highly similar results (see Tables S4 and S5 in the Supplemental Material). h2 = communality. Boldface highlights loadings ≥ |.4|.aLoadings on this factor were multiplied by –1.




model (Costa & McCrae, 1992). Bonobos with higher scores for the Attentiveness factor thus tended to be more focused and perhaps better at solving problems. On the other hand, bonobos with low scores for this factor tended to be easily distracted and uncoordinated.

Agreeableness in bonobos resembled its counterparts in orangutans (Weiss et al., 2006) and chimpanzees (King & Figueredo, 1997). It also included loadings associated with high sociability in mountain gorillas (Eckardt et al., 2015), all of which were also associated with high agree-ableness in humans (Costa & McCrae, 1992). Some of the items were associated with high agreeableness in Hanuman langurs (Konecná et al., 2008), high extraver-sion in humans (Costa & McCrae, 1992) and extraversion in chimpanzees (King & Figueredo, 1997). Bonobos with high scores for this factor thus tended to empathize with and help their conspecifics, whereas bonobos that had low scores for this factor did not.

Extraversion in bonobos differed from dimensions labeled extraversion, sociability, or friendliness in other species (Eckardt et al., 2015; King & Figueredo, 1997; Konecná et al., 2008; Morton et al., 2013; Schaefer & Steklis, 2014; Weiss et al., 2006, 2007, 2009, 2011). The items “solitary,” “depressed,” “autistic,” and “individualistic” all had negative loadings on this factor. Traits such as “soli-tary” and “depressed” are associated with low extraversion in humans (Costa & McCrae, 1992) and chimpanzees (King & Figueredo, 1997), low sociability in brown capuchin monkeys (Morton et al., 2013), and low friendliness in rhe-sus macaques (Weiss et al., 2011). The item “autistic” is associated with high neuroticism in chimpanzees (Weiss et al., 2009) and low sociability in brown capuchin mon-keys (Morton et al., 2013). The item “individualistic” is associated with extraversion in chimpanzees (Weiss et al., 2009) and components or factors labeled sociability (Eckardt et al., 2015) or extraversion (Schaefer & Steklis, 2014) in gorillas. Thus, compared with bonobos who had low scores in this factor, those with high scores tended to be well integrated into their social environment.

We also tested the extent to which these dimensions were dependent on the use of exploratory factor analy-sis. We therefore extracted six components using princi-pal component analysis. The varimax- and promax-rotated structures derived via principal component analysis (see Tables S4 and S5 in the Supplemental Material) were nearly identical to the varimax- and promax-rotated structures derived via factor analysis.

Factor reliabilities

Interrater reliabilities, internal consistencies, and test-retest reliabilities for the factors are shown in Table 2. Interrater reliability in the first wave was fair for attentive-ness and agreeableness and excellent for the remaining factors. Interrater reliabilities of these factors in the sec-ond wave were all excellent. Internal consistencies were excellent, except for extraversion in the first wave. All factors showed good test-retest reliability except for agreeableness.

Interspecies comparisons

We used targeted orthogonal Procrustes rotations (McCrae et al., 1996) to compare the personality structures of bono-bos and chimpanzees (see Table 3). For all three compari-sons, the total congruence, which is based on all of the factor or component loadings (McCrae et al., 1996, p. 559), indicated that, overall, the factor or component structures for all three chimpanzee samples exhibited a fair degree of similarity with the bonobo structure. The pattern of congru-ence for the factors or components was consistent across comparisons of the structures derived from the three chim-panzee samples to the structure derived from the bonobo sample: The highest congruence coefficients were found for assertiveness, conscientiousness, openness, and agree-ableness, and the lowest congruence coefficients were found for attentiveness and extraversion. Specifically, asser-tiveness, conscientiousness, and agreeableness exhibited at

Table 2. Reliabilities and Test-Retest Reliabilities of the Factors

Factor

Wave 1 reliability Wave 2 reliability Test-retest reliability

ICC(3,1) ICC(3,k) α ICC(3,1) ICC(3,k) α r p

Assertiveness .64 .83 .92 .72 .88 .95 .62 < .0001Conscientiousness .61 .81 .90 .59 .81 .94 .61 < .0001Openness .60 .80 .87 .65 .84 .91 .37 < .01Attentiveness .23 .45 .84 .52 .76 .90 .41 < .01Agreeableness .32 .55 .87 .54 .77 .92 .17 > .05Extraversion .61 .81 .69 .67 .86 .83 .28 < .05

Note: Significant test-retest reliabilities were still significant when we used permutation tests. The intraclass correlation coefficients (ICCs) indicate (a) the reliability of individual ratings of factor scores, ICC(3,1), and (b) the reliability of mean ratings of factor scores, ICC(3,k).



1436 Weiss et al.

least fair similarity in the ChimpanZoo and Yerkes chim-panzee samples and fair similarity in the Japanese chim-panzee sample. In addition, for openness, the congruence coefficients indicated fair similarity in the ChimpanZoo and Japanese samples; the congruence coefficients for the Yerkes sample did not indicate similarity. Extraversion in the ChimpanZoo sample exhibited a fair degree of similar-ity to bonobo extraversion; the congruence coefficients for this factor in the Yerkes sample and Japanese sample, how-ever, were not indicative of similarity. Finally, no factors or components similar to attentiveness were found in any of the chimpanzee samples.

Discussion

We found that 54 personality traits in bonobos were defined by the factors Assertiveness, Conscientiousness, Openness, Attentiveness, Agreeableness, and Extraversion. These factors showed interrater reliabilities and test-retest reliabilities comparable to those of human personality factors (e.g., Costa & McCrae, 1992) and personality traits and dimensions in other species (Bell, Hankison, & Laskowski, 2009; Gosling, 2001). Of these factors, asser-tiveness, conscientiousness, agreeableness, and possibly openness had chimpanzee analogues. In contrast, the evidence for chimpanzee analogues of extraversion and attentiveness was weak. This pattern of results suggests that four bonobo factors (assertiveness, conscientious-ness, agreeableness, and openness) were ancestral and present in the common ancestor of chimpanzees and bonobos and that two bonobo factors (extraversion and attentiveness) were derived, possibly as a result of differ-ences in bonobo and chimpanzee socioecology and bonobo social behavior (Furuichi, 2011; Hare et al., 2012; Vervaecke et al., 2000).

The bonobo Assertiveness factor was similar to chim-panzee dominance (King & Figueredo, 1997; Weiss et al., 2007, 2009) and to components and factors identified in other primate species (Eckardt et al., 2015; Konecná et al., 2008; Konecná et al., 2012; Morton et al., 2013;

Schaefer & Steklis, 2014; Weiss et al., 2006, 2011). This finding calls into question the perception that bonobos are egalitarian (de Waal, 1995) and the predictions made by some that bonobos should not have a dominance or assertiveness dimension (Eckardt et al., 2015). However, it is consistent with behavioral observations, which reveal strong dominance hierarchies in captive bonobos (Stevens, Vervaecke, de Vries, & van Elsacker, 2007; Vervaecke et al., 2000).

Items defining conscientiousness components or fac-tors in chimpanzees and humans (King & Figueredo, 1997; Weiss et al., 2007, 2009) loaded on two bonobo factors. The first factor, Conscientiousness, was character-ized by high levels of predictability and low levels of impulsive aggression. It thus somewhat resembled the deliberation facet of conscientiousness in humans (Costa & McCrae, 1992) and the proto-agreeableness compo-nent in mountain gorillas (Eckardt et al., 2015). The sec-ond factor, Attentiveness, was characterized by the ability to focus on and do well in carrying out tasks. It thus resembled the order and self-discipline facets of consci-entiousness in humans (Costa & McCrae, 1992) and the attentiveness component in brown capuchin monkeys (Morton et al., 2013). This split of conscientiousness traits into two factors may explain bonobos’ superior perfor-mance compared with chimpanzees on tests related to social causality (Herrmann, Hare, Call, & Tomasello, 2010). Further testing should reveal the degree to which this factor reflects cognitive skills in bonobos.

The bonobo Openness factor resembles that of chim-panzees, except that items related to activity and playful-ness, which load on extraversion in chimpanzees (King & Figueredo, 1997; Weiss et al., 2007, 2009), load on the Openness factor in bonobos. A similar pattern has been found in gorillas (Eckardt et al., 2015) and capuchin mon-keys (Morton et al., 2013). This difference between bono-bos and chimpanzees may reflect the fact that, in bonobos, play is used to reduce tension (Palagi, 2006) and has an important role in trust building (Behncke, 2015).

The bonobo Agreeableness factor resembled that of chimpanzees (King & Figueredo, 1997; Weiss et al., 2007, 2009), humans (Costa & McCrae, 1992), orangutans (Weiss et al., 2006), western gorillas (Gold & Maple, 1994), and Hanuman langurs (Konecná et al., 2008). Conversely, the Extraversion factor in bonobos was dis-similar to that in chimpanzees, because it characterized individuals that were not withdrawn and depressed. A somewhat similar pattern was found for the sociability component in mountain gorillas (Eckardt et al., 2015).

When Eckardt et al. (2015) tried to explain the evolu-tionary bases for differences between the personalities of mountain gorillas and chimpanzees (which were similar to those we found between chimpanzees and bonobos), they hypothesized that distinct extraversion, openness,

Table 3. Congruence Coefficients Calculated by Rotating Chimpanzee Structures Toward the Bonobo Structure

Chimpanzee sample

Factor ChimpanZoo Yerkes Japan

Assertiveness .947 .855 .896Conscientiousness .953 .956 .921Openness .947 .811 .905Attentiveness .656 .787 .781Agreeableness .905 .909 .927Extraversion .853 .685 .782 Total .906 .852 .874




and agreeableness dimensions would benefit primate species characterized by fission-fusion social structures, because they would offer individuals more flexibility to cope with constantly changing social environments than if these factors were not distinct. The presence of sepa-rate Openness, Agreeableness, and Extraversion factors in bonobos is consistent with this hypothesis; bonobo social structure is considerably more flexible than that of gorillas (Robbins, 2011). Although direct comparisons of the degree of fission-fusion in different ape species have been hindered by differences in methodology and study duration (Aureli et al., 2008), researchers working on future comparative studies may wish to test whether dif-ferences in social-structure stability across species are associated with the structure of traits related to extraver-sion, openness, and agreeableness.

This study is not without limitations. One limitation is that the sample size was relatively small for factor analy-sis; another is that translation issues may have affected ratings of bonobos housed in German zoos. Therefore, the differences between bonobos and chimpanzees may not be species differences but may instead reflect sam-pling variability. However, the pattern of congruence between the current bonobo sample and the three chim-panzee samples was highly consistent. A third limitation is that this study was limited to captive bonobos as opposed to wild bonobos, which may have different per-sonality structures. However, this feature of our study ruled out the possibility that differences between bono-bos and chimpanzees could reflect differences between captive and wild populations. Nonetheless, these limita-tions suggest there would be much value in collecting additional data, both via questionnaires and by behav-ioral observations and tests, on bonobos and chimpan-zees housed in zoos, sanctuaries, and in the wild.

Although there was considerable overlap between the personalities of bonobos and chimpanzees, most probably reflecting their recent common ancestor, there were differ-ences, too, that may be the signals of more recent selec-tion. Although more work needs to be done in this area, we expect that it is bound to reveal an even more com-plete picture of the evolution of personality in Hominidae.

Author Contributions

A. Weiss and N. Staes contributed equally to the manuscript and should be considered co-first authors. A. Weiss, J. J. M. Pereboom, and J. M. G. Stevens developed the study concept. All authors contributed to the study design. N. Staes, J. J. M. Pereboom, and J. M. G. Stevens collected the data on bonobos, and A. Weiss and N. Staes analyzed all the data. A. Weiss and M. Inoue-Murayama collected data on chimpanzees in Japan, and A. Weiss contrib-uted data from the ChimpanZoo and Yerkes samples. A. Weiss and N. Staes drafted the manuscript, and the other authors pro-vided suggestions and critical revisions. All authors approved the final version of the manuscript for submission.

Acknowledgments

Data collection would not have been possible without the par-ticipation of the zoological institutions, sanctuaries, and research institutes. We also thank Jim King, Virginia Landau, Bill Hopkins, and Tetsuro Matsuzawa for their help in obtaining chimpanzee ratings and Sara Van Coillie for translating the questionnaire and for assisting in collecting and entering data. We also thank Jeff McCrae for answering a question about Procrustes rotation and Dick Byrne for comments on an earlier draft.

Declaration of Conflicting Interests

The authors declared that they had no conflicts of interest with respect to their authorship or the publication of this article.

Funding

This work was supported by funding from the University of Antwerp (to M. Eens); University of Edinburgh Development Trust Grant 2828 (to A. Weiss); Daiwa Foundation Small Grant 6515/6818 (to A. Weiss); Japanese Ministry of Education, Culture, Sports, Science and Technology Grants-in-Aid for Science Research 25290082 and 25118005 (to M. Inoue-Murayama); and funding from the Cooperation Research Program of the Primate Research Institute, Kyoto University (to M. Inoue-Murayama). Support for the Center for Research and Conservation of the Royal Zoological Society of Antwerp was provided by the Flemish Government. The collection of chimpanzee data at Yerkes National Primate Research Center was supported by National Institute of Neurological Disorders and Stroke Grants NS36605 and NS42867 and by National Center for Research Resources Grant RR00165 (all to the Yerkes National Primate Research Center). The content of this publication is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health or the U.S. Government.

Supplemental Material

Additional supporting information can be found at http://pss .sagepub.com/content/by/supplemental-data

Open Practices

Releasing the data publicly could violate the confidentiality of the raters and the facilities that house the bonobos. The data are available from the corresponding author. The complete Open Practices Disclosure for this article can be found at http://pss .sagepub.com/content/by/supplemental-data.

Note

1. The English-language version of the HPQ for chimpanzees can be found at http://extras.springer.com/2011/978-1-4614-0175-9/weiss_chimpanzee_personality.pdf

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Personality in Bonobos

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